Spatial variation in anthropogenic mortality induces a source–sink system in a hunted mesopredator

Abstract

Lethal carnivore management is a prevailing strategy to reduce livestock predation. Intensity of lethal management varies according to land-use, where carnivores are more intensively hunted on farms relative to reserves. Variations in hunting intensity may result in the formation of a source-sink system where carnivores disperse from high-density to low-density areas. Few studies quantify dispersal between supposed sources and sinks-a fundamental requirement for source-sink systems. We used the black-backed jackal (Canis mesomelas) as a model to determine if heterogeneous anthropogenic mortality induces a source-sink system. We analysed 12 microsatellite loci from 554 individuals from lightly hunted and previously unhunted reserves, as well as heavily hunted livestock- and game farms. Bayesian genotype assignment showed that jackal populations displayed a hierarchical population structure. We identified two genetically distinct populations at the regional level and nine distinct subpopulations at the local level, with each cluster corresponding to distinct land-use types separated by various dispersal barriers. Migration, estimated using Bayesian multilocus genotyping, between reserves and farms was asymmetric and heterogeneous anthropogenic mortality induced source-sink dynamics via compensatory immigration. Additionally some heavily hunted populations also acted as source populations, exporting individuals to other heavily hunted populations. This indicates that heterogeneous anthropogenic mortality results in the formation of a complex series of interconnected sources and sinks. Thus, lethal management of mesopredators may not be an effective long-term strategy in reducing livestock predation, as dispersal and, more importantly, compensatory immigration may continue to affect population reduction efforts as long as dispersal from other areas persists.

Not exodus, but population increase and gene flow restoration in Cantabrian brown bear (Ursus arctos) subpopulations. Comment on Gregório et al. 2020

“…According to most studies, human-caused mortality produces home range takeover in carnivores. Dispersal of individuals from low to high mortality areas (compensatory immigration) has been documented for different carnivore species, in which unevenly distributed hunting pressures induce source–sink dynamics [ 16 – 19 ]. In the case of bears, Lamb et al [ 20 ], using 41 years of demographic data for more than 2,500 brown bears in the Rocky Mountains, concluded in a recent publication that population growth rates for bears in human dominated areas revealed a source-sink dynamic.…”

Borrowing from Peter to pay Paul: managing threatened predators of endangered and declining prey species

“…In the case of endangered species this might result in the collapse of prey populations on a large scale (Johnson, Isaac & Fisher, 2007). Removing dominant individuals from a population is inherently spatially and socially disruptive (Tuyttens et al, 2000; MacDonald, Riordan & Mathews, 2006; Loveridge et al, 2009; Davidson et al, 2011; Minnie et al., 2018). Furthermore, translocation has yet to be shown as a viable option, due to the territorial nature of pinnacle carnivores and their intense intra- and interspecific competitiveness.…”

Social organization, home ranges, and extraterritorial forays of black‐backed jackals

“…Because of extirpations of large carnivores, black‐backed jackals are the dominant predator over most of South Africa, including on small livestock farms where domestic sheep are often their main prey (Kamler et al ; Drouilly et al ; Minnie et al 2016, ). Consequently, jackals are heavily persecuted on small livestock farms because of their real or perceived predation on livestock (Du Plessis et al ; Minnie et al , Drouilly et al ).…”

“…Consequently, jackals are heavily persecuted on small livestock farms because of their real or perceived predation on livestock (Du Plessis et al ; Minnie et al , Drouilly et al ). In contrast, on many nature reserves, jackals are not hunted, or are hunted at a much lower intensity compared to livestock farms (Kamler et al , Drouilly et al , Minnie et al ). This has created a source‐sink system of jackal populations in South Africa based on anthropogenic mortality, whereby reserves act as source populations of jackals for surrounding livestock farms, which act as sinks (Minnie et al ).…”

“…In contrast, on many nature reserves, jackals are not hunted, or are hunted at a much lower intensity compared to livestock farms (Kamler et al , Drouilly et al , Minnie et al ). This has created a source‐sink system of jackal populations in South Africa based on anthropogenic mortality, whereby reserves act as source populations of jackals for surrounding livestock farms, which act as sinks (Minnie et al ). Furthermore, jackals living on some reserves may go on forays to surrounding livestock farms, apparently because of the abundance of food there (Kaunda ); sheep and other livestock have been reported in scats of jackals living in several different reserves (Bothma , Rowe‐Rowe 1976, , Kaunda and Skinner ).…”